title: Evaluation of age-dependent susceptibility in calves infected with two doses of Mycobacterium avium subspecies paratuberculosis using pathology and tissue culture.
abstract: The longstanding assumption that calves of more than 6 months of age are more resistant to Mycobacterium avium subspecies paratuberculosis (MAP) infection has recently been challenged. In order to elucidate this, a challenge experiment was performed to evaluate age- and dose-dependent susceptibility to MAP infection in dairy calves. Fifty-six calves from MAP-negative dams were randomly allocated to 10 MAP challenge groups (5 animals per group) and a negative control group (6 calves). Calves were inoculated orally on 2 consecutive days at 5 ages: 2 weeks and 3, 6, 9 or 12 months. Within each age group 5 calves received either a high - or low - dose of 5 × 10⁹ CFU or 5 × 10⁷ CFU, respectively. All calves were euthanized at 17 months of age. Macroscopic and histological lesions were assessed and bacterial culture was done on numerous tissue samples. Within all 5 age groups, calves were successfully infected with either dose of MAP. Calves inoculated at < 6 months usually had more culture-positive tissue locations and higher histological lesion scores. Furthermore, those infected with a high dose had more severe scores for histologic and macroscopic lesions as well as more culture-positive tissue locations compared to calves infected with a low dose. In conclusion, calves to 1 year of age were susceptible to MAP infection and a high infection dose produced more severe lesions than a low dose.
authors.affiliation: Department of Ecosystem and Public Health, University of Calgary, Hospital Drive NW, Calgary, AB, Canada. jdebuck@ucalgary.ca.
authors.forename: Rienske A R
authors.initials: RA
authors.lastname: Mortier
authors.forename: Herman W
authors.initials: HW
authors.lastname: Barkema
authors.forename: Janet M
authors.initials: JM
authors.lastname: Bystrom
authors.forename: Oscar
authors.initials: O
authors.lastname: Illanes
authors.forename: Karin
authors.initials: K
authors.lastname: Orsel
authors.forename: Robert
authors.initials: R
authors.lastname: Wolf
authors.forename: Gordon
authors.initials: G
authors.lastname: Atkins
authors.forename: Jeroen
authors.initials: J
authors.lastname: De Buck
mesh_terms: D000367:Age Factors; D000818:Animals; D002417:Cattle; D002418:Cattle Diseases / Q000235:genetics / Q000382:microbiology*; D015169:Colony Count, Microbial / Q000662:veterinary; D004198:Disease Susceptibility / Q000382:microbiology / Q000662:veterinary; D005260:Female; D008297:Male; D016927:Mycobacterium avium subsp. paratuberculosis / Q000502:physiology*; D010283:Paratuberculosis / Q000235:genetics / Q000382:microbiology*; D011897:Random Allocation; D060888:Real-Time Polymerase Chain Reaction / Q000662:veterinary